The Provence side moved on to 75 points, an unassailable eight-point lead over second-placed Lille with two games left, triggering wild celebrations in the football mad city.
At the Stade Velodrome, Rennes striker Jimmy Briand cancelled out Gabriel Heinze's early opener before the break, but Marseille struck back late in the second half.
Senegal striker Niang made it 2-1 in the 76th minute and Argentine midfielder Lucho Gonzalez wrapped it up two minutes later.
"It rewards nine months of hard work," coach Didier Deschamps, who was the last OM captain to lift the national trophy, told French TV channel Orange Sport.
"The first half was not easy for us, with all the expectations weighing on our shoulders."
Marseille, stripped of the title in 1993 following a match-fixing scandal the same year they won the Champions League, also won the League Cup this season but the title had an even sweeter taste.
Fans dived in the sea at the Old Port minutes after the final whistle was blown despite the driving rain and low temperatures in southern France.
"I was too young to enjoy Marseille's last titles so tonight I'm going to celebrate hard," Thomas, a 21-year-old fan, told Reuters, his OM scarf wrapped around his neck.
Police officers were patrolling the city's main streets as thousands of fans chanted 'We are the champions, we are the champions'.
Heinze opened the scoring with a 25-metre free kick in the fourth minute as OM got off to a perfect start in a capacity stadium.
Rennes, however, controlled the game and Briand grabbed a deserved equaliser seven minutes from the break.
The French international, linked with a move to Marseille next season, headed home from Asamoah Gyan's cross after he was left unmarked by Heinze.
LONG WAIT
Two minutes before the interval, Valbuena found Niang in the box with a defence splitting pass, but keeper Nicolas Douchez dived at the Senegal striker's feet.
Niang was pushed in the box by France defender Rod Fanni on the hour but referee Ruddy Buquet did not award the penalty.
Deschamps replaced Brandao with Hatem Ben Arfa and the move spurred OM back into the game.
Douchez failed to block a Valbuena strike and Niang, who had followed up, fired home from close range.
Lucho put the result beyond doubt two minutes later after benefiting from excellent work by Ben Arfa on the left flank, the Argentine's curling shot from just outside the box effectively sealing Marseille's ninth French league title.
Earlier, Lille moved up to second with a 2-0 win at Toulouse, leapfrogging AJ Auxerre after the Burgundy side slumped to a 2-1 defeat at fourth-placed Olympique Lyon.
Lyon, who grabbed all three points when Bosnia midfielder Pjanic netted with three minutes left, stayed fourth on 65 points and they have a game in hand.
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